1. Field of the Invention
This invention relates to a telecommunications access network, and in particular to an optical fibre telecommunications access network.
2. Related Art
In the United Kingdom, the telecommunications network includes a trunk network which is substantially constituted by optical fibre, and a local access network which is substantially completely constituted by copper pairs. Flexibility in the copper access network is provided at two points en route to the customer; firstly, at street-side cabinets serving up to 600 lines; and secondly, at distribution points (DPs) serving around 10-15 lines. Eventually, it is expected that the entire network, including the access network, will be constituted by fibre.
The ultimate goal is a fixed, resilient, transparent telecommunications infrastructure for the optical access network, with capacity for all foreseeable service requirements. One way of achieving this would be to create a fully-managed fibre network in the form of a thin, widespread overlay for the whole access topography, as this would exploit the existing valuable access network infrastructure. Such a network could be equipped as needs arise, and thereby could result in capital expenditure savings, since the major part of the investment will be the provision of terminal equipment on a `just in time` basis. It should also enable the rapid provision of extra lines to new or existing customers, and flexible provision or reconfiguration of telephony services.
In order to be future proof, the network should be single mode optical fibre, with no bandwidth limiting active electronics within the infrastructure. Passive optical networks (PONs) offer total transparency and freedom for upgrade.
The most common optical network is the simplex single star, with point-to-point fibre for each transmit and receive path, from the exchange head end (HE) to the customer network terminating equipment (NTE). This network design involves high fibre count cables, and unique electro-optic provision at HE and NTE for each customer. The resulting inherent cost can only be justified for large business users, who generally also require the security of diverse routing, which increases the cost still further.
The advent of optical splitters (power dividers) allows the power transmitted from a single transmitter to be distributed amongst several customers, thereby reducing and sharing the capital investment. In 1987, BT demonstrated this technology in a system for telephony on a passive optical network (TPON), with a 128-way split and using time division multiplex (TDM) running at 20 Mb/s. This combination enabled basic rate integrated service digital network (ISDN) to be provided to all customers. In practice, the competitive cost constraint of the existing copper network precludes domestic customers from having just telephony over fibre, due to the high capital cost of equipment. This may change in the future. In the meantime, telephony for small business users (for example those having more than 5 lines) can probably break this barrier.
The wider range of services and higher capacity required by business customers makes a 32-way split more attractive for a 20 Mb/s system, and this has been demonstrated by BT's local loop optical field trial (LLOFT) at Bishop's Stortford.
In summary, the use of splitter-based PON architecture will reduce the cost of fibre deployment in the access network. When compared with point-to-point fibre, savings will result from:
(i) reducing the number of fibres at the exchange and in the network; PA0 (ii) reducing the amount of terminal equipment at the exchange; PA0 (iii) sharing the cost of equipment amongst a number of customers; PA0 (iv) providing a thin, widespread, low cost, fibre infrastructure; and PA0 (v) providing a high degree of flexibility, and allowing `just in-time` equipment and service provision.
Additionally, PON architecture can be tailored to suit the existing infrastructure resources (duct and other civil works).
Total network transparency will retain the option for future services to be provided on different wavelengths to the telecommunications, which for TPON is in the 1300 nm window. By transmitting at other wavelengths, other services, such as broadband access for cable television and high definition television, or business services, such as high bit rate data, video telephony or video conferencing, can be provided. The huge bandwidth potential of fibre promises virtually unlimited capacity for the transparent network. Eventually, it may be possible to transmit hundreds of wavelengths simultaneously, as the development of technology in optical components, such as narrow band lasers, wavelength division muitiplexers (WDMs), optical filters, fibre amplifiers and tuneable devices, moves forward.
For this potential to remain available, and for the access network to be used to provide many and varied services, then it must be designed and engineered to provide very high levels of security and resilience. Even for simple POTS (plain old telephony service), advance warning and live maintenance are essential to limit disruption.
A particularly important aspect of resilience is the reliability of the network, and this is particularly the case for business customers. Thus, even if a business customer has several lines, these will tend to come from the same DP, so that a failure between that DP and the upstream cabinet, or a failure between that cabinet and the exchange, or even a failure at the exchange itself will result in loss of service to that customer.
The problem of resilience is discussed in Commutation et Transmission, Vol. 14, No. 4, 1992, Paris, FR, pages 27-34, J. Abiven et al, MOLENE:systeme de distribution d'acces a 2 M bit/s sur reseau optique passif", and Commutation et Transmission, Vol. 15, No. 1, 1993, Paris, FR, pages 5-10, D. Berlinet et al: Researux optiques flexibles", wherein resilience is achieved with the provision of a dependent, duplicated backup network.